As is explained in T. H. James, The Theory of the Photographic Process, 4th Edition, published by Macmillan, pages 393-394, fog forms in the areas which are not exposed to light under the non-selective action of the developer. Fog may be defined as the density obtained in the parts which have not been exposed to light. Fog may be due to the nature of the emulsion, to the developing, to oxidation by air or to aging.
It is known that fog may be promoted by an increase in sensitivity and it is essential therefore that a reduction in the level of fog does not take place at the cost of a loss of sensitivity.
Normally anti-fogging agents are used to reduce or eliminate the formation of fog. In particular mineral anti-fogging agents such as mercury II salts have been used. For example, European patent 0 352 618 describes the use of a combination comprising a mercury oxide or salt and a benzothiazolium compound for exerting a stabilizing and anti-fogging action.
These mercury derivatives have drawbacks with regard to the environment. Organic compounds or their salts are also used as anti-fogging or stabilizing agents. For example, U.S. Pat. No. 2,694,716 describes the use of salts of polymethylene-bis-benzothiazolium as anti-fogging agents in photographic emulsions. These compounds are not as effective as mercury salts.
Thus the problem of the fog of emulsions is not always satisfactorily resolved and it would be very desirable to be able to have emulsions with characteristics such that the presence of anti-fogging agents would be superfluous.
The aim of the present invention is a process for obtaining an emulsion of the core/shell type having a low fog level when the emulsion has just been prepared or after aging, an emulsion which also has a good compromise between fog and sensitivity. Core/shell emulsion is taken to mean an emulsion in which the silver halides are distributed non-uniformly within the grain. A conventional method for preparing such emulsions consists of forming grains with a multilayer structure by successively precipitating regions with different halide compositions. The central region is normally referred to as the core and the other regions are grouped together under the name shell.
Many patents of the prior art describe bromoiodide emulsions of the core/shell type in which the proportion of iodide varies between the central region and the various regions forming the shell, the central region having the highest proportion of iodide and the outermost region having the lowest proportion of iodide.
U.S. Pat. No. 4,777,564 describes a high-sensitivity photographic emulsion consisting of silver bromoiodide grains with a total silver iodide content of at least 12% molar and in which the iodide is distributed within the grains in at least three regions of silver bromoiodide having different iodide contents, the outermost region having an iodide content less than the total iodide content of the grain. For example, the outermost region has an iodide concentration of 5 to 15% and the central region an iodide concentration of 30 to 70%. This patent gives no information about fog.
U.S. Pat. No. 4,990,437 describes a silver bromoiodide emulsion containing 1 to 20% molar iodide and consisting of 3 to 8 regions. The central region consists of silver bromide or silver bromoiodide. The central region comprises 0 to 40% molar iodide and the outermost region 0 to 10% molar iodide. Between the outermost region and the layer of the shell having the highest iodide content is an intermediate layer having an iodide content which is intermediate with respect to the two layers surrounding it. This intermediate layer has a composition which can be uniform within the layer or else may have a gradient. Amongst the additives in the emulsion are the conventional organic anti-fogging agents and stabilizers.
U.S. Pat. No. 4,444,877 describes a silver bromoiodide emulsion of the core/shell type in which the central region comprises 0.5 to 10% molar iodide preferably distributed uniformly, and the shell does not include any iodide and is preferably a bromide shell. Conventional organic anti-fogging agents are used with this type of emulsion.
U.S. Pat. No. 4,565,778 describes core/shell emulsions consisting of two regions. The difference in iodide concentration between the two regions should be not less than 1% molar; the shell has an iodide content close to 0. The central region may consist of two or more layers having different iodide contents. Although it is said that the iodide content within the grain may vary continuously between two layers, in all the examples the iodide content varies abruptly. These emulsions are used with the conventional organic anti-fogging agents. They have a higher sensitivity and contrast than reference emulsions having a uniform iodide distribution. The sensitivity is improved without increasing the fog by effecting a chemical ripening in the presence of a labile selenium compound.
European patent 0 430 196 describes a process for stabilizing the crystalline morphology of tabular grains of AgCl when the emulsion is subjected to heat treatment such as the treatments used in chemical sensitization for example. In order to obtain these tabular grains containing at least 50% chloride, the central silver chloride region is formed by the double jet method and, into the jet of chloride, a second halide is then introduced in an evenly increasing profile, for example bromide in order to form a chlorobromide shell.
U.S. Pat. No. 4,668,614 describes bromoiodide emulsions of the core/shell type comprising a central region consisting of 10 to 45% molar iodide and a shell consisting of one or two layers, the outermost layer comprising less than 5% molar iodide. The total iodide concentration is 7% molar or more. The shell preferably consists of silver bromide. These emulsions are used in the presence of conventional organic anti-fogging agents. The examples show that they have a low fog level and a higher sensitivity than reference emulsions which do not have two separate regions with very different iodide contents.
Examples in this patent show that different iodide distributions in the shell and core make it possible to have improved sensitivity without increasing fog compared with an emulsion comprising the same amount of iodide distributed uniformly in the grain. However, the problem of fog is resolved only partially since the emulsions in this patent are used with the conventional organic anti-fogging agents.
It can be seen that, in the patents of the prior art describing the preparation of core/shell emulsions, the iodide concentration is generally uniform in the central region but there may be a concentration gradient in the shell. In general, the high iodide gradient at the center of the grain decreases towards the outside of the grain through a succession of regions having a well-defined iodide content. The problem of fog remains, requiring the presence of anti-fogging agents.